The goal of this research is to understand the molecular basis of gamete interactions in fertilization, using the mouse as a model for mammalian fertilization. The first specific binding events between mammalian gametes occur at the surface of the zona pellucida (ZP). Mouse sperm bind to ZP3, a glycoprotein of this egg associated matrix, and this interaction signals the sperm to undergo the acrosomal exocytosis, which is required for successful fertilization. Biochemical analysis has determined that there are approximately 30,000 ZP3 receptors per sperm and biophysical analysis has indicated that only a few high affinity receptors would be required to bind their ligand for successful sperm adhesion. The number of ZP3 receptors required to signal acrosomal exocytosis has not been determined. Many cell types produce significantly greater numbers of receptors than required to generate the maximum cellular response to the signal. The function of the """"""""unused"""""""" fraction of receptors, the receptor reserve, is not well understood. It seems unlikely that a cell would expend resources on superfluous components and, therefore, various theories have addressed putative functions of the receptor reserve. The receptor reserve may function to ensure cell responses at low ligand concentrations, to provide an optimum receptor density for localized signalling, to provide efficient receptor-effector coupling, to increase the probability of cell responsiveness, or to reduce the latency of cellular response. The objective of the current study is to establish what fraction of the ZP3 receptor population is both necessary and sufficient to induce acrosomal exocytosis, what fraction constitutes the receptor reserve, and what the function of the receptor reserve in modifying the kinetics of signalling acrosomal exocytosis may be. The proposed studies will use quantitative assays to calculate receptor occupancy and an established histological assay to quantify sperm response (acrosomal exocytosis). The results will yield novel information constituting a major advance not only in our understanding of sperm-egg interactions during mammalian fertilization but also in our understanding of the potential functions of receptor reserve in signalling systems. Understanding the mechanisms regulating the acrosome reaction is essential for our understanding of fertilization and for developing effective alternatives to control fertilization.